With advances in drilling technology it is currently possible to drill horizontal wellbores deep into hydrocarbon producing reservoirs. There are many advantages to a horizontal well completion including extending contact with a producing formation thereby facilitating drainage and production of the reservoir.
Although horizontal wellbores allow more contact with the producing formation, one encounters some difficulties in well completions associated with horizontal wellbores not commonly dealt with in vertical wells. One area of concern in well completions is the inability to effectively clean out the horizontal section so production casing can be run into the well and cemented. Another area of concern is the inability to effectively clean a cased wellbore in conjunction with completion or workover operations. Present methods leave drilled solids, e.g., formation fines and cuttings, in the hole which contaminates cement and interferes with bonding and strength of cement as the well is completed.
The kind of contaminant remaining in a wellbore depends of course on the operation previously conducted therein. Composition of fluids used in previous operations can vary. Drilling muds, completion fluids, and workover fluids have varying compositions.
Drilling mud are those fluids used in rotary drilling to cool and lubricate the bit, lift cuttings from the borehole, and to control borehole pressure. Drilling muds should have a viscosity, density, and a fluid retention level suitable for the particular drilling application and the formation being drilled. For instance, a drilling mud generally must be sufficiently dense to control the pressure of the well and simultaneously sufficiently plastic to carry and lift cuttings from the well.
Completion fluids are those fluids used after drilling is complete and during the steps of completion of the well. Completion can include cementing the casing, perforating the casing, setting the tubing and pump, etc.
Workover fluids are those fluids used during remedial work in the well. This can include removing tubing, replacing a pump, cleaning out sand or other deposits, reperforating, etc. Workover also broadly includes steps used in preparing an existing well for secondary or tertiary oil recovery such as polymer additions, micellar flooding, steam injection, etc., or for well stimulation such as acidizing, fracturing the formation, etc.
If problems involving the composition of the various fluids used in a wellbore weren't enough, there is the added problem of air pockets within a horizontal wellbore which prevent portions of a wellbore from being contacted with a cleaning fluid. Therefore, what is needed is a method for cleaning a horizontal wellbore which is effective in removing various contaminants and which can also contact previously precluded portions of said wellbore.